diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
index c06cf1aa38..00f48d190f 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
@@ -83,20 +83,12 @@ private[http] object HttpServerBluePrint {
   def controller(settings: ServerSettings, log: LoggingAdapter): BidiFlow[HttpResponse, ResponseRenderingContext, RequestOutput, RequestOutput, Unit] =
     BidiFlow.fromGraph(new ControllerStage(settings, log)).reversed
 
-  def requestPreparation(settings: ServerSettings)(implicit mat: Materializer): BidiFlow[HttpResponse, HttpResponse, RequestOutput, HttpRequest, Unit] = {
-    val prepareRequest =
+  def requestPreparation(settings: ServerSettings)(implicit mat: Materializer): BidiFlow[HttpResponse, HttpResponse, RequestOutput, HttpRequest, Unit] =
+    BidiFlow.fromFlows(Flow[HttpResponse],
       Flow[RequestOutput]
         .splitWhen(x ⇒ x.isInstanceOf[MessageStart] || x == MessageEnd)
         .via(headAndTailFlow)
-        .via(requestStartOrRunIgnore(settings))
-        // FIXME #16583 / #18170
-        // `buffer` is needed because of current behavior of collect which will queue completion
-        // behind an ignored (= not collected) element if there is no demand.
-        // `buffer` will ensure demand and therefore make sure that completion is reported eagerly.
-        .buffer(1, OverflowStrategy.backpressure)
-
-    BidiFlow.fromFlows(Flow[HttpResponse], prepareRequest)
-  }
+        .via(requestStartOrRunIgnore(settings)))
 
   def requestStartOrRunIgnore(settings: ServerSettings)(implicit mat: Materializer): Flow[(ParserOutput.RequestOutput, Source[ParserOutput.RequestOutput, Unit]), HttpRequest, Unit] =
     Flow.fromGraph(new FlowStage[(RequestOutput, Source[RequestOutput, Unit]), HttpRequest, Unit]("RequestStartThenRunIgnore") {